Congenital Disorders of Glycosylation (CDG) are rare inherited defects in sugar chain (glycan) synthesis and their addition to protein. All 14 types (different genes) of CDG-I patients have mutations in different genes, but share a common lesion: lack of full N-glycosylation site occupancy. Likewise, patients share many symptoms, but show broad clinical variations both within and between different types. There are no vertebrate animal models for these disorders and only one Type, CDG-Ib, has a therapy. Here we propose to analyze the first viable CDG-I mouse model, evaluate the single known therapy, and apply that therapy to currently untreatable types of CDG. CDG-Ib patients have insufficient phosphomannose isomerase (MPI, Fru-6-P_>Man-6-P) activity and develop liver dysfunction, fibrosis, failure to thrive, protein-losing enteropathy and coagulopathy. However, dietary supplements of mannose bypass the defect by increasing the flux of Mannose through a minor biosynthetic pathway (Man`Man-6-P) thus relieving nearly all symptoms. CDG-Ia patients, who are deficient in PMM2 (Man-6-P`Man-1-P), do not respond to mannose therapy because they catabolize Man-6-P via robust MPI. Note that PMM2 and MPI compete for the same critical substrate, Man-6-P, and their ratio determines its metabolic flux. We engineered a hypomorphic Mpi allele to create the first viable potential CDG-I mouse model. Analysis of these mice to date shows progressive hepatopathology and increased enteric protein loss. In Aim 1 we will determine whether these mice show pathology modeling CDG-Ib patients. Aim 2 will determine how hypomorphic lines respond to environmental stresses. Aim 3 will determine whether mannose rescues (prevents and/or reverses) the susceptibility of hypomorphic mice to pathology, specifically, protein-losing enteropathy and hepatic pathology. Since the various CDG types share many of the pathologies, successful treatment of one type may establish a paradigm to treat others. Therefore, in Aim 4 we will breed our Mpi-hypomorphic mice with Pmm2-deficient mice that currently die in utero. Based on highly encouraging preliminary data, we predict that providing mannose and genetically reducing Mpi activity in Pmm2-deficient mice will rescue the lethal phenotype by increasing the metabolic flux of Man-6-P into the depleted glycosylation pathway. If successful, this approach will show that redirecting mannose flux into the depleted glycosylation pathway has therapeutic potential and would empower high-throughput screening search for mannose flux-enhancing compounds for several types of CDG.